Cryptococcus neoformans causes a lethal meningoencephalitis in 8 percent of AIDS patients in the US. Current treatment is inadequate as 10-20 percent of patients die from cryptococcal meningitis despite aggressive antifungal therapy, and individuals who survive beyond the initial treatment period must be maintained on life-long therapy to prevent relapse. Because of these therapeutic limitations, antibodies have been considered as prevention and treatment for C. neoformans infection. Anti-capsular monoclonal antibodies can prolong the life of lethally infected mice and increase the effectiveness of antifungal agents in vivo. Previous observations indicate that functions mediated by the constant regions of these antibodies are crucial in determining their protective potential. Consistent with this idea are preliminary studies we have done in genetically deficient mice showing that antibody interactions with certain Fc receptors (FcRs) are important in mediating protection, while the presence of complement may be detrimental to antibody efficacy, particularly in the absence of FcR binding. Over the past several years, we have developed a large library of recombinant antibodies with a variety of different functional properties. We will now graft anti-cryptococcal variable regions onto these antibodies to examine, both alone and in combination, the contribution of such characteristics as FcR binding, complement activation, avidity and half life to efficacy in an animal model of infection with C. neoformans. We will first confirm the functional properties of these antibodies in vitro by testing FcR binding, antibody-dependant cell mediated cytotoxicity (ADCC), and complement activation and determine their pharmacokinetics in vivo. We will then test in vivo efficacy against infection with C. neoformans. Specifically, we will investigate whether antibody efficacy depends on i) in vivo persistence, ii) ability to activate complement, iii) engagement of FcRs or iv) effective cross-linking of surface antigen. The experiments proposed here are designed to explore the hypothesis that antibodies with different constellations of functional properties will have differing degrees of efficacy. Murine infection with C. neoformans is a particularly relevant system because a murine monoclonal antibody is currently undergoing Phase I evaluation for the treatment of cryptococcal meningitis in AIDS patients. If this trial proves promising, the experiments proposed will provide the basis for selecting the best therapeutic candidates for further study. A clear definition of what properties correlate with efficacy should allow us to move forward in designing effective therapeutic antibodies for treatment of disease in humans.